The "Mini" checkbox does not appear in quickpress, using WP 3.1 and Miniposts 0.6.13
I'm not sure at all, but it seems that changing:
if ($frame['function'] == 'wp_dashboard_quick_press') {
to:
if ($frame['function'] == 'wp_dashboard_quick_press_output') {
in miniposts.php solved the problem.
May someone else check this fix is OK and relevant?